The present invention relates to a method of polishing the surface of glass articles.
After being manufactured, glass articles display surface defects which are of two kinds:
defects inherent in their manufacture, referred to as defects to a macroscopic scale, such as traces of mould joints, sharp edges which should be smoothed down, accidental markings such as tool impacts, etc.
defects referred to as being of a microscopic scale, such as roughness, minute cracks, etc., which defects impair the appearance of the glass article obtained (brilliancy, transparency, etc.) and its mechanical properties (compressive strength, . . . ).
The technique employed in glass manufacture to eliminate these defects is the polishing method which permits improving the surface condition of the article by causing the disappearance of unevennesses and of the other defects.
Amongst the industrial processes for polishing glass articles in use until now, may be mentioned mechanical polishing and chemical polishing. The mechanical polishing operation is performed by means of successive passes of the glass article over ever finer abrasive wheels. This process enables the production of articles of high quality but on the other hand it is very costly. Furthermore, it can only be utilised for articles having a particular geometry, such as bottles produced for high-grade perfumery goods, cut-glass goods, etc. The chemical polishing operation consists in immersing the glass article in an acid bath, most frequently of hydrofluoric acid. This immersion in an acid bath causes the superficial destruction of the siliceous supporting lattice and the elimination of cations, in particular of alkaline cations. This treatment however amounts to no more than a violent cleaning or scouring action and is not applicable to all kinds of glass. Furthermore, it raises many problems in respect of its application and environment, considering the chemical reactants utilised.
Tests were attempted some time ago for polishing glass articles by means of an oxygen-hydrocarbon flame, in particular of oxygen and natural gas, as is disclosed in particular in U.S. Pat. No. 3,188,190. In accordance with these tests, the glass article is caused to pass in front of the oxygen-hydrocarbon flame; this causes a superficial remelting action and the filling of the cracks. In theory, this flame polishing operation should have rendered it possible to obtain a satisfactory surface quality, since all the bonds have time to be normalised at high temperature. Unfortunately, during these tests, a collapse of the article was observed, occurring before an adequate surface quality was obtained, because of excessive in-depth heating. The tests of this nature were actually successful only with glass articles of very great thickness.